For aqueous emulsion polymerization of monomers to form rubbery polymers, disproportionated rosin acids in the form of alkali salts thereof are typically utilized in the emulsification system. The disproportionated rosin acid salt generally serve a dual purpose, first as an emulsion aid during the polymerization and, secondly, in the polymer after its recovery from the polymerization system as a compounding additive in the preparation of a compounded rubber.
The disproportionated rosin acid salt itself is typically converted to the rosin acid form as a result of the acidic condition of the coagulation or isolation step for recovering the rubber from the emulsion or latex.
Thus, the rosin is usually present in the emulsion as the salt and present in the recovered rubber as the acid.
It is considered necessary, in the aqueous emulsion polymerization process, that the rosin acid or salt thereof be used in a disproportionated form so that its conjugated double bonds are essentially eliminated and thus substantially prevented from interfering with the emulsion polymerization system. The disproportionation of the rosin acid can be typically accomplished by heating the acid in the presence of a catalyst to modify its double bonds, namely, to eliminate their conjugated configuration.
For solution polymerization of monomers to form rubbery polymers where an emulsion is not desired, disproportionated rosin acids have been used directly as a shortstop rather than adding the alkali salt thereof. In such application, the rosin acid is added to the polymerization system at a desired time to stop the polymerization reaction as compared to the utilization of the salt form of the acid during an emulsion polymerization as an emulsifying aid.
However, it should be appreciated that the use of such disproportionated rosin acid for these purposes in a solution polymerization can add an appreciable cost to the recovered rubber itself.
Therefore, it is desired to provide an effective shortstop means for a solution polymerization system in order to prepare a rubber composition, as well as a resultant tire containing such a rubber composition.